Villa had led 2-0 until the 76th-minute when Manchester City stormed back, scoring three goals in a span of just five minutes to complete the comeback.
After the victory earned Manchester City its sixth Premier League title in 11 years, home supporters at Etihad Stadium stormed the pitch in celebration.
During the invasion, Olsen was apparently captured on video getting hit on the back of the head as he was being escorted off the field.
“Those idiots who attacked me will not destroy my emotions of today’s game,” he later wrote on Instagram. “I got the opportunity to make my debut for this fantastic club, and we were really close to get a good result against a tough opponent.”
Aston Villa manager Steven Gerrard was also angry over the incident. During a press conference, he told reporters that questions about the situation should go to Manchester City before he abruptly left to check on Olsen.
Manchester City manager Pep Guardiola later apologized to both Olsen and Aston Villa.
“I’m so sorry on behalf of the club,” he told reporters. “The emotions … it’s always difficult to understand why the people respond in this way. Hopefully, we can discover who it was, and he’ll be punished.”
Manchester City also issued a statement over the incident.
“Manchester City would like to sincerely apologize to Aston Villa goalkeeper Robin Olsen, who was assaulted after the final whistle at today’s match when fans entered the pitch,” the statement read.
“The Club has launched an immediate investigation and once identified, the individual responsible will be issued with an indefinite stadium ban,” it added.